The success of North Korea’s first intercontinental ballistic missile, possibly capable of reaching the US state of Alaska, has sparked speculation over how the isolated country managed to advance the technology so much faster than expected.

Experts said available images of Pyongyang’s missiles showed obvious Russian traits, but most likely they were developed with the Norths own technology developed over decades.

“All the missiles so far revealed by Pyongyang could be clearly traced back to a couple of older Russian models they acquired decades ago,” said Zhao Tong, ­a fellow in the nuclear policy programme at the Carnegie-Tsinghua Centre for Global Policy.

That included the latest Hwasong-14 tested on July 4 with an estimated range of more than 6,000km, which qualifies as an intercontinental ballistic missile.

Its liquid-fuel engine – as well as that of its predecessor, the 3,700km-range Hwasong-12 – originated from an old Soviet R-27 Zyb submarine-launched ballistic missile (SLBM), Zhao said.

The R-27 technologies are thought to have come into Pyongyang’s possession in 1992. The chaos after the Soviet Union’s 1991 collapse provided then North Korean leader Kim ­Il-sung, Kim Jong-un’s grandfather, with the chance to obtain the SLBM, which can carry a ­nuclear warhead and has a range of 2,400km.

Sun Xingjie, a Korean politics expert at Jilin University, said that as early as the cold war era, North Korea had made assembling a ­nuclear weapon a fundamental state strategy, and had received significant help from the Soviet Union and later Russia. “All three generations of leaders of the Kim family, especially Kim Jong-un lately, put a tremendous amount of resources into the research and development,” Sun said.

The USSR provided its communist ally with education and training, expert advisers and even blueprints, according to Zhao. North Korea initially based its primary missile development in the 1970s on Soviet Scud missiles. From there, it developed its early Rodong and Taepodong rockets.

Generations of North Korean engineers have been sent to study at Moscow’s nuclear and missile research institutions such as the Joint Institute for Nuclear ­Research in Dubna. These technicians went on to become key contributors to Pyongyang’s weapons development programme.

Andrei Chang, founder of military magazine Kanwa Asian ­Defence, said a key factor behind Pyongyang’s fast progress was that its missile experts were not only well-educated and bright, but were in the habit of “working hard without complaint”.

“Scholars from Russia’s military academies told me that in the 1980s missile and engineering ­department, the most excellent and hardworking students were all from North Korea,” he said.

The regime also hired a number of Russian experts after the fall of the USSR. In 1992, it was reported that a group of Russian scientists and missile specialists were arrested while attempting to travel to Pyongyang. But many Russian engineers were already working in North Korea then, ­according to a report by the Nuclear Threat Initiative think tank.

“It is likely that through long research, the North Korean engineers made a major breakthrough over some key technological difficulties,” Zhao said. “But I don’t see any indication that North Korea has acquired important new technology in recent years.”

Pyongyang’s recent advances include successful tests of the Pukguksong-1 and -2, which use solid-fuel engines more advanced than the liquid-fuel R-27.

International observers do not completely agree on the origin and development path of these missiles. There was speculation that the Pukguksong-1, an SLBM, might have been developed with help from China’s SLBM. But pictures of the Pukguksong launches show the designs also bear strong resemblance to the R-27.

“From what we’ve seen, North Korea worked out the technology of solid-fuel engines by repeatedly copying the Russian liquid-fuel engines. Based on that they developed the solid-fuel,” Song Zhongping, a military analyst, said. “No significant similarity to the ­Chinese models can be found in North Korean missiles. The reason is simple – China doesn’t want North Korea to possess and develop advanced missiles.”

Pakistan, an ally of Beijing, is also unlikely to have passed on any missile technology from China to North Korea, although they did cooperate on nuclear weapons for some time before the United Nations imposed sanctions on Pyongyang, he said.

North Korea has defied international objections to its nuclear weapons programme, despite the sanctions. Pyongyang was determined to become a “leading missile power in Asia”, Song said.

North Korea has no claim to that status yet, as its R-27 technologies, developed by the Soviets from 1968 to 1988, still trail those in the most advanced missiles from the United States, Russia, China, and even India and Israel.

But Pyongyang has exchanged missile technology with Iran, and helped Tehran with its missile and rocket development. The Hwasong missile family and Iran’s later Shahab series are obviously related. Some R-27 technologies have been also seen on Iranian rockets Safir and Simorgh.

UN sanctions and the trade embargo on missile-related electronic devices and fuel-related chemical substances has forced Pyongyang to find illegal smuggling channels or to use civilian products as substitutes, hindering its progress on ICBMs, Song said.

But the Hwasong-14 could likely reach Alaska and Hawaii, and it was only a matter of time before North Korea developed a nuclear warhead that could be loaded on it, Chang said. “They’re moving closer to doing it.”